1. Field of the Invention
The present invention relates to the production of monochromatic light, and in particular relates to a monochrometer for use in an analytical centrifuge.
2. Description of Related Art
The following United States Patents are incorporated by reference herein: U.S. Pat. Nos. 4,830,493; 4,919,537; and 4,921,350. These patents were issued to the inventor of the present invention and were commonly assigned to Beckman Instruments, Inc., the assignee of the present invention. The patents describe monochrometers designed for use in analytical centrifuges.
A monochrometer is a device used to supply a collimated beam of light having some desired, narrow range of wavelengths. A monochrometer typically has the following component parts: (1) an entrance slit with a source of radiation of a wide range of wavelengths; (2) a prism or diffraction grating dispersing the incident radiation into a continuous spectrum of wavelengths; (3) some mechanism to rotate the prism or grating so that the desired spectrum of radiation is obtained; and (4) an exit slit selectively isolating a narrow band of wavelengths. For spectrophotometry studies, a detector is positioned at the monochrometer exit for detecting the radiation antennuation of a sample placed between the exit and the detector. Appropriate signal amplification circuit is provided in conjunction with the detector.
Referring to FIG. 1, the arrangement of a prior art monochrometer in an analytical centrifuge as described in the patents is briefly summarized. The centrifuge 10 comprises a rotor 12 driven to rotate about an axis by a motor 14. The rotor 12 has several sample cells 16 having transparent windows to allow a monochromatic light beam from the monochrometer 18 to be directed vertically through each sample cell 16 as the cells rotate pass the beam. A detector 20 is positioned below the rotor 12 in line with the beam. As illustrated in FIG. 1, the monochrometer 18 comprises a light tube 22 folded along its length. Light from a source 24 is directed through the light tube 22 to a mirror 26 having diffraction rulings. The mirror 26 can be tilted to direct light of a particular spectrum range through the remaining length of the light tube towards the rotor. A slit (not shown) at the tube exit select a narrow wavelength band from the spectrum for transmission through the sample cells 16.
The prior art monochrometer has certain limitations in obtaining a true monochromatic beam of light. Specifically, the diffraction grating mirror reflects light at wavelengths other than that of interest, including higher order wavelengths. The light exiting through the slit at wavelengths other than that of interest is often referred to as "stray light". The stray light is scattered at various angles which reflects or scatters off the internal walls of the light tube 22. Some of the reflected or scattered stray light is incident back to the diffraction grating mirror 26 which causes additional diffractions at the wrong wavelengths or is mixed in with the monochrometer output. The reliance upon a monochrometer output of a known wavelength of light is however critical to the results of the spectrophotometric study.